Immunosenescence, or the decrease in immune system function with aging, is a major contributor to the general decrease in the quality of human health associated with aging. A primary cause of aging-related immunodeficiency is the postnatal involution of the thymus with age, which results in decreased production of naive T cells. The central importance of thymic involution in the development of immunosenescence is well established;however, the cellular and molecular mechanisms by which this process occurs are less clear. Identification of these mechanisms is of clear clinical relevance, as they represent potential therapeutic targets for inducing thymic rebound and immune system regeneration, and also have significance for cell replacement strategies. One mechanism that has been well-documented is the role of androgen depletion in inducing rebound - surgical or chemical castration in humans or mice induces at least transient thymic recovery and increased T cell production, and deletion of the intracellular Androgen Receptor (AR) causes increased thymus size. Since androgens primarily act by affecting gene expression within a receiving cell via AR, identifying which cell(s) receive the androgen signal relevant to involution is critical to understanding the mechanism by which androgens affect the thymus. However, as data from castration and the Ar null mice are complicated by the systemic effects of global androgen depletion or insensitivity, neither the cellular target nor the mechanism by which rebound occurs in these models is known. Furthermore, no thorough study of the Ar null thymus during natural aging has been published. Thus, both the normal role of androgens in the lifespan of the thymus and the mechanism by which androgen depletion affects thymus function are uncertain. The available data consistently point to a thymic stromal target as the major site of androgen action in the thymus. Among these stromal cell types, thymic epithelial cells (TECs) are the leading candidates to mediate the effects of androgen signaling. In the current collaborative proposal, we will test the hypothesis that the effects of androgens on the thymus are mediated by direct effects on thymic stroma, in particular TECs. Furthermore, we propose that these effects are, in part, mediated directly or indirectly by the TEC-specific transcription factor FoxN1. To this end, we propose two specific aims: 1) to test the hypothesis that androgens are primary regulators of thymic involution and rebound by comparing the effects of systemic and tissue-specific androgen receptor deletion;and 2) to test the molecular mechanisms by which androgen signaling acts in the postnatal thymus. In the proposed experiments we will use molecular and genetic approaches to investigate the mechanisms by which androgens affect the postnatal thymus, and by which thymic rebound is mediated. These studies may identify novel targets for therapeutic intervention in the treatment of immunosenescence and thymic involution. PUBLIC HEALTH RELEVANCE: Immunosenescence is an important contributor to increased incidence of disease and reduced quality of life with aging. A key component of immunosenescence is thymic involution, but the mechanisms underlying this process are poorly understood. Steroid hormones, especially androgens, are thought to be major contributors to involution, and androgen blockade can result in thymic regeneration. The current proposal seeks to identify the cellular and molecular targets of androgen signaling in the postnatal thymus and during thymic rebound.